The manufacture of high density, narrow line width integrated circuits requires the use of high resolution lithography equipment. The early photolithographic techniques used ultraviolet or natural light to expose the patterns on the wafer. However, ultraviolet and natural light techniques have resolution limitations. In particular, diffraction, interference, and light divergence are common; they cause a reduction in resolution and they limit the circuit yield per wafer. In the case of very complex integrated circuits (e.g. VLSI), the size of the components forming the circuits approaches the wavelengths used to produce the masks (around 1 .mu.m) and large geometric errors can thereby arise. The resolution ultimately obtained in the resist is thus limited by, among other factors, the wavelength of the incident light.
In part because of these disadvantages, X-ray lithography was developed to take advantage of the shorter wavelengths of soft X-rays to expose appropriate patterns in the resists. The wavelength of the X-rays, which generally ranges from about 0.1 to 1.0 nanometers, significantly improves the resolution and circuit yield per wafer associated with lithography.
During X-ray lithography, an X-ray source such as a synchrotron is used to direct an intense collimated beam of X-rays through an X-ray mask overlying a photoresist layer of a semiconductor wafer. The mask includes a central, X-ray transparent region with selected patterns formed of X-ray absorbing material. The X-rays expose patterns on the underlying photoresist layer that correspond to the apertures in the mask formed by the X-ray absorbing material. The resist layer is then developed in the normal fashion to provide a pattern on the surface of the semiconductor wafer. X-ray lithography offers the advantages of improved resolution combined with a large depth of field, vertical walled patterns, and simplicity in forming the circuit patterns on the semiconductor wafers.
As materials for absorber patterns, it is required to use materials which sufficiently absorb soft X-rays. If the wavelength of a soft X-ray is determined, the quantities of soft X-rays absorbed by an X-ray absorber can easily be calculated based on an X-ray absorption coefficient. Elements of higher atomic numbers such as gold, tantalum, tungsten, rhenium or the like must be used in order to obtain an X-ray attenuation of the. order of 10 dB, so that a sufficient degree of mask contrast is attained.
In practice, only gold is commonly used as an absorber material. When Ta, W, Re or the like, which have high melting points, are deposited in the form of thin films, high stresses are produced so that the thin mask substrate is damaged and distorted. Therefore, gold, which is relatively easily processed, is preferred. When gold is used as an absorber material, the gold film must have a thickness of about 0.52 .mu.m for A1--K radiation (8.34 .ANG.), about 0.68 .mu.m for Si--K radiation (7.13 .ANG.) and about 0.20 .mu.m for Cu--L radiation (13.2 .ANG.) in order to obtain a mask contrast of 10 dB.
There are two conventional processes for providing gold absorbers. In one process, an insulating film is defined to form an electroplating mask and gold is plated on the mask substrate. The other process, not of interest here, is the ion etching method. The former process, in which fine gold patterns are electroplated through a mask of insulating material, can produce submicron patterns having steep profiles. control of the plating process is essential for repeatably achieving high-quality absorber films with low in-plane distortion.
The characteristics of the plated Au films can affect the x-ray mask in the following ways: (1) Stress of the film--films plated under high stress may delaminate from the substrate. They may also distort the mask membrane. (2) Plating uniformity--the thickness of the film should be the same across the entire plated area. (3) Morphology--it is desirable to keep the grain size small and surface roughness at a minimum in the plated films. Otherwise, thickness variations can produce excessive and variable x-ray attenuation across a particular pattern. (4) Reproducibility--plating results should be reproducible from run to run.
The use of arsenite (AsO.sub.2.sup.-) in gold plating baths is known. Sel-Rex.RTM. BDT.RTM. 510 gold electroplating process solution (OMI International Corp., Nutley, N.J.) is an example of a plating solution based on the gold sulfite anion complex. The manufacturer's recommendation is to use 10 mL of "brightener" solution per gallon of gold plating solution. The nature of the brightener solution is not disclosed in the manufacturer's technical data sheet for BDT.RTM. 510; arsenic analysis indicates that it contains sufficient AsO.sub.2.sup.- to provide 58 .mu.g/mL of AsO.sub.2.sup.- in the final plating solution. We have found that 58 .mu.g/mL produces films that exhibit much too high tensile stress for submicron rule integrated circuits. (See FIG. 9 below)
Thus there is a need for a gold deposition process that reproducibly provides a uniform layer of sufficient thickness (0.6 to 0.7 .mu.m) and morphology to provide reasonable contrast but without high stress.